In positive working photosensitive lithographic printing plates which have hitherto been widely used, only exposed portions of photosensitive layers are mainly removed by dissolution with developing solutions. In this case, non-image areas are generally developed with alkaline developing solutions mainly containing silicates having hydrophilization action.
This is for preventing the non-image areas from being stained with ink during printing by hydrophilization of the non-image areas with the silicate-containing alkaline developing solutions.
However, when the developing ability is somewhat decreased by exhaustion of developing solutions and developing replenishers, variation with the elapse of time of the positive working photosensitive lithographic printing plates, or insufficient exposure, the photosensitive layers of portions to form the non-image areas are not sufficiently removed by dissolution with the developing solutions, which causes residues of photosensitive layer components to remain on the plates in some cases.
When the photosensitive layer components remain on the non-image areas in the developing process as described above, the non-image areas can not be sufficiently hydrophilized only by the use of the silicates, resulting in the phenomenon of allowing ink to adhere to the non-image areas in the course of printing, namely the problem of developing scum.
Then, the present inventors have developed intermediate layers containing the polymers described in JP-A-10-69092 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") as a method for retaining the adhesion between aluminum supports to which sufficient hydrophilicity is given and photosensitive layers during printing, and sufficiently removing the photosensitive layers and the intermediate layers by dissolving them with alkaline developing solutions in the non-image areas, thereby realizing the compatibility of press life with scumming prevention. This improves the adhesion of image areas, and in the non-image areas, the photosensitive layers and the intermediate layers can be sufficiently removed with the alkaline developing solutions. Accordingly, surfaces of the hydrophilized supports are exposed, and scumming prevention performance can also be maintained.
According to this method, however, deletion performance (image removability in removing unnecessary images using chemical agents generally called deletion fluids) is not sufficient, and particularly, significantly deteriorates with an increase in the area of images to be deleted. Further, there is the problem that binders contained in the photosensitive layers remain on the non-image areas of the supports, which causes the scum during printing (hereinafter referred to as photosensitive layer removability). It has been therefore desired to improve it.